You need to have a valid purchase code to be able to get an API key for the theme registration.
To obtain the purchase code:
1Go to ThemeForest and login into your account.
2In the upper right corner, hover over your username and click on Downloads
3Scroll down to Jupiter, click on Download button, and then click on the “License certificate & purchase code (text)” link.
4In the text file, take note of the Item Purchase Code. It is similar to rt76ed7sd-0000-1234-9655-111111111111